1. Field of the Invention
The present invention relates to a large diameter wafer conveying system, and more particularly, to a large diameter wafer conveying system in which the structure of a wafer holder for holding a wafer by vacuum absorption is modified in such a manner that the operation thereof can be stabilized, and which can prevent the wafer from being damaged by a residual vacuum when it is unloaded.
2. Description of the Related Art
Generally, in the semiconductor manufacturing process, extreme care should be taken when handling a wafer. For example, when the wafer is conveyed several times for sequential processes, including etching, diffusing, oxide layer growing, depositing, testing, and assembling, etc., the wafer is apt to be damaged due to inadvertent handling. The wafer may be damaged by scratches or by being broken into pieces, which critically affects yield.
Referring now to FIG. 1, a conventional wafer conveying system comprises a wafer holder 1 mounted on a conveying mechanism (not shown), a vacuum supplier 3 for supplying the wafer holder 1 with vacuum through a vacuum hose 2, and a valve unit 4 for switching the vacuum supplier 3 ON or OFF. As more clearly seen in FIGS. 2 and 3, the wafer holder 1 also includes several tweezers 1a for holding a wafer by vacuum absorption, and a tweezer block 1b for fixing the tweezers 1a.
Each tweezer 1a has a linear configuration, and the front end of each tweezer 1a is rounded to prevent the wafer from being damaged when it is loaded or unloaded. The tweezers 1a are vertically arranged, and the rear ends of the tweezers 1a are fixed to the tweezer block 1b, in order to convey several wafers simultaneously.
In the conventional wafer conveying system constructed as described above, when the valve unit 4 is turned ON to convey the wafer to a boat or a carrier, vacuum is supplied from the vacuum supplier 3 to the front end of the tweezer 1a, whereby the wafer is absorbed onto the tweezer 1a. If the valve unit 4 is turned OFF, the vacuum is removed to unload the wafer from the tweezer 1a.
However, the wafer conveying system constructed as mentioned above can only be applied to wafers having a diameter not greater than 8 inches. Accordingly, considering the trend toward larger diameter wafers, the conventional wafer conveying system has many drawbacks as discussed further below.
First, since the tweezer 1a of the wafer holder 1 absorbs and holds only one wafer, the larger the wafer, the larger must be the tweezer 1a to securely hold the wafer. If the wafer is not totally absorbed and held by the tweezer 1a at its center, there is a possibility that the horizontal balance of the wafer will be affected when conveying it, such that the wafer would be scratched or broken by the wall of a slot of the carrier, thereby lowering yield. Also, if the size of the tweezer 1a is not sufficiently large when the tweezer 1a holds a larger diameter wafer, for example, greater than 12 inches, the wafer will tend to sag and overlap the other wafer which has already been loaded in the carrier, thereby causing the wafer to be scratched, or generate contaminating articles, or even be broken.
Second, in the wafer conveying system which uses vacuum absorption for holding a wafer, even when the valve unit 4 is turned OFF to unload the wafer o n the boat or the carrier by removing the vacuum, there remains a residual vacuum in the wafer holder 1, causing the wafer to remain attached to a boat slot, resulting in a large amount of contaminating particles being transferred to other wafers disposed downwardly therefrom, deteriorating their quality.